Technical Field
The present invention relates to an infrared imaging device, and more specifically relates to an infrared imaging device including an infrared detector that detects incident infrared rays and converts the detected incident infrared rays into an electrical signal. In addition, the present invention relates to a method of updating fixed pattern noise data in such an infrared imaging device.
Background Art
Infrared imaging devices that detect incident infrared light (infrared rays) and generate an infrared image have been known. Generally, the infrared imaging devices include an infrared detector that detects infrared rays radiated from a subject and converts the detected infrared rays into an electrical signal. The infrared imaging devices are used in a wide range of fields such as a monitoring camera, a night vision device, thermography, and a front monitoring device mounted in a vehicle, an airplane or the like.
The infrared imaging device generates fixed pattern noise specific to the device such as a variation in the sensitivity of the infrared detector, or variations in the gain and offset of a circuit. Particularly, in the infrared imaging device using a focal plane array arranged two-dimensionally as infrared detector elements, each detector element within the array has a fluctuation in characteristics. Therefore, as a result, fixed pattern noise changing for a relatively long time is generated.
The generation of the fixed pattern noise causes the occurrence of a fluctuation in a pixel value and leads to a uniform image not being obtained even in a case where an image of a surface having uniform temperature is captured by the infrared imaging device. In order to reduce the influence of the fixed pattern noise, data of the fixed pattern noise (fixed pattern noise data) may be acquired, and the fixed pattern noise data may be subtracted from an image signal obtained by capturing an image of a subject. The fixed pattern noise data is acquired, for example, in a state where a light source having a uniform amount of light is installed at the front of the infrared detector and infrared rays incident on the infrared detector are cut off from the outside.
Since the fixed pattern noise fluctuates depending on a change in the environment such as temperature, there is a demand for the fixed pattern noise data to be repeatedly acquired in the course of imaging. In order to meet such a demand, a technique is proposed in which a shutter mechanism is provided within an infrared imaging device, and infrared rays incident on an infrared detector are cut off from the outside to acquire fixed pattern noise data (see, for example, JP1998-142065A (JP-H10-142065A)). However, in JP1998-142065A (JP-H10-142065A), the shutter mechanism unnecessary for original imaging is required to be disposed in the periphery of an optical system and the infrared detector, which leads to an increase in cost or an increase in the size of a device. In addition, there is also a disadvantage of an increase in the number of failure occurrence points.
JP2001-336983A discloses an infrared imaging device capable of acquiring fixed pattern noise data while a shutter mechanism is not required. The infrared imaging device disclosed in JP2001-336983A includes an optical system that condenses light radiated from a target object (subject) to cause the condensed light to be incident on an infrared detector, and a subtractor that outputs image data obtained by subtracting fixed pattern noise data from an image signal (image data) which is output by the infrared detector. The optical system is positioned at a focusing location by a focus adjustment mechanism during a normal imaging.
In JP2001-336983A, the optical system is controlled to be in a non-focused state during the acquisition of the fixed pattern noise data. The optical system is set to be in a non-focused state, light from all directions within an observation field view of the optical system is uniformly incident on the infrared detector. That is, a flux of light incident on the optical system from various directions is not imaged at a specific point of the infrared detector, and is uniformly incident on the detection surface of the infrared detector. In this state, an error between output image data of the subtractor and expected value data of the fixed pattern noise is obtained. A negative feedback given to the subtractor is performed using return data based on this error as the fixed pattern noise data, and the return data when the output image data of the subtractor and the expected value data are set to be substantially the same as each other is held as the fixed pattern noise data.
In JP2001-336983A, image data obtained in a state where a light source having a uniform amount of light is installed at the front of the infrared detector and infrared rays incident on the infrared detector are cut off from the outside is used as the expected value data of the fixed pattern noise. The optical system is set to be in a non-focused state, and light from all directions within an observation field view is caused to be incident on the detection surface of the infrared detector. Thereby, it is possible to cause infrared rays having a uniform amount to be incident on each detector element, and to obtain the fixed pattern noise data without using the shutter mechanism.